Self-healing concrete: what ancient Roman concrete can teach us

Concrete is an incredibly useful and versatile building material on which not only today's societies, but also the ancient Roman Empire were built. To this day, Roman concrete structures can be found in mundane places such as ports, but also the Pantheon in Rome, which forms the largest unreinforced concrete dome in existence to date at 43.3 meters in diameter. and is in excellent condition despite being nearly 1,900 years old.

Even as the Roman Empire fell and retreated into what became the Byzantine Empire - also known as the Eastern Roman Empire - and the world around these last remnants of Roman architecture changed and changed again, all these concrete structures remained despite the knowledge of how to build structures like these being lost in the ages. Perhaps the most amazing thing is that even today our concrete is not as durable, despite modern inventions such as reinforcement with rebar.

The reverse engineering of ancient Roman concrete has been the source of intense study and debate for decades, with a recent paper by Linda M. Seymour and colleagues adding an important clue to the puzzle. Could "hot mixing", with pockets of reactive lime clasts inside hardened concrete, provide self-healing properties?

At the heart of modern concrete and mortar is cement: it is the binder that is mixed with additional ingredients such as sand and gravel to turn it into concrete. Although every type of cement starts with calcium carbonate (CaCO3), how this base material is processed and mixed determines its hardening and other properties of the material that will be exhibited. The most basic type is called non-hydraulic cement, which begins with the firing of calcium carbonate (also called limestone), which produces calcium oxide and carbon dioxide:

CaCO3 → CaO + CO2

CaO is commonly referred to as quicklime, as well as burnt lime. In the next step, this quicklime is mixed with water to “quench” it:

CaO + H2O → Ca(OH)2

When the resulting calcium hydroxide is then exposed to carbon dioxide, the cement will begin to harden:

Ca(OH)2 + CO2 → CaCO3 + H2O

At atmospheric concentrations of carbon dioxide, this is a very slow process and therefore generally impractical for construction. Common types of cement such as Portland cement are therefore hydraulic cements, which harden by reacting with the clinker minerals that are part of the mix. Portland cement is a fine powder composed of at least two-thirds calcium silicates, primarily alite (Ca3SiO5) and belite (Ca2SiO4), plus additional silicates and aluminates.

The first step in making hydraulic cement is the same as for non-hydraulic cement, with the firing of the calcium carbonate:

CaCO3 → CaO + CO2

The next step is where things get interesting, as the resulting calcium oxide reacts with silicates and aluminates:

2 CaO + SiO2 → 2 CaO · SiO2

3 CaO + SiO2 → 3 CaO · SiO2

3 CaO + Al2O3 → CaO · Al2O3

Finally, the oxides (calcium oxide, aluminum oxide and ferric oxide from e.g. brownmillerite) react to form the final hydraulic cement mix:

4CaO + Al2O3 + Fe2O3 → 4 CaO · Al2O3 · Fe2O3

Basically, this is the recipe that all hydraulic cement uses, be it "Roman cement", Portland cement or ancient Roman cement. When used to create concrete, this cement is mixed with aggregates, usually small rocks, stones and sand. When adding water to this mixture, the hydration process will cause...